Characteristics and reducibility of nanosized La1-XCexCoO3 perovskites synthesized by reactive grinding - Nguyen Tien Thao

552 Journal of Chemistry, Vol. 47 (5), P. 552 - 557, 2009 Characteristics and reducibility of nanosized La1-xCexCoO3 perovskites synthesized by reactive grinding Received 7 June 2008 NGUYEN TIEN THAO1, NGO THI THUAN1, SERGE KALIAGUINE2 1 Faculty of Chemistry, College of Sciences, Vietnam National University, Hanoi 2 Department of Chemical Engineering, Laval University, Canada. G1K 7P4. Abstract Two (La,Ce)CoO3 perovskite-type oxides prepared by reactive grinding have been characterized by means of X-ray diffraction, N2 adsorption/desorption isotherms, SEM, H2-TPR. Both samples show a nanocrystalline phase and rather high surface area. The thermal stability of cobalt ions in such solids has been studied under reducing conditions. H2-TPR results indicate that the reduction of Co3+ to Co0 occurs in a multiple-step. The partial replacement of La3+ by Ce4+ has promoting effects on the reducibility of cobalt ions. A decreased reduction temperature of cobalt ions in the presence of Ce4+ leads to an increased dispersion of the reduced phase for La0.9Ce0.1CoO3 perovskite precursor. Keywords: perovskite; reduction; cobalt; La-Ce-Co; dispersion; metal. I - Introduction Perovskite-type oxides with the general formula ABO3 have been much paid special attention for the last three decades due to their potential applications in catalysis and pollutant elimination [1]. For example, some perovskites such as LaCoO3, LaMnO3, LaCrO3 have been used in environmental catalysis reactions including CO oxidation, methane combustion, NOx decomposition, and exhaust treatment [1- 2]. However, these applications are mainly based on the electronic configuration of the B3+ cations. In recent years, there has been an interest in using mixed-oxide type perovskites Ln(Co,Fe, Ni, Cu)O3 as prominent precursors for preparation of a “metal on oxide” solid [3- 5]. Because of the complexity of perovskite system which often contains mixed valence ions and variable stoichiometry, a transition metal with lower oxidation states could exhibit numerous different kinds of catalytic sites active for several reactions when pre-treated under tailored conditions [4 - 6]. For example, the reduction of La(Co,Fe,Ni,Cu)O3 perovskites at high temperature leads to the formation of a blend of metals, alloys, ions. [3 - 6]. The reduced phase was reported to be very active for hydrogenation of ethylene [7], reforming of CO2 [4], Fischer-Tropsch synthesis [3], alcohol synthesis [5, 8, 9]. In some ways, the incomplete reduction of perovskite results in the formation of the intermediates of transition metals and the homogeneity of active sites on the catalyst surface which may become a promising the catalytic sites for many appropriate reactions. This part of the present study is to deal with the reducibility of cobalt ions in two ground perovskites and to pave the way for preparation 553 of several catalytic systems from the cobaltate perovskite precursors. The next contributions will be concentrated on the applications of the incomplete reduction of La1-xCexCoO3 perovskites in the epoxidation of styrene and in the hydrogenation of carbon monoxide. II - Experimental La1-xCexCoO3 perovskite-type mixed oxides were synthesized by the reactive grinding method [5, 8, 10]. The stoichiometric proportions of commercial lanthanum, cerium, and cobalt oxides (99%, Aldrich) were mixed together with three hardened steel balls (diameter = 11 mm) in a hardened steel crucible (50 ml). A SPEX high energy ball mill working at 1000 rpm was used for mechano-synthesis. Milling was carried out for 8 hours prior to a second milling step with an alkali additive if used. A sample was added into a beaker containing 1200 mL water and stirred by magnetic stirring for 90 min prior to being sedimented for 3-5 hours. After the clean water is removed, the slurry was dried in oven at 60 - 80oC before calcination at 250oC for 150 min. The specific surface area of all obtained samples was determined from nitrogen adsorption equilibrium isotherms at -196oC measured using an automated gas sorption system (NOVA 2000; Quantachrome). X-ray diffraction (XRD) was collected using a SIEMENS D5000 diffractometer with CuKα radiation (λ = 1.5406 nm). Bragg’s angles between 20 and 60o were collected at a rate of 1o/min. Average crystal domain sizes (D) were evaluated by means of the Debye-Scherrer equation D = Kλ/βcosθ after Warren’s correction for instrumental broadening [11]. SEM images were recorded using a JEOL JSM-840 electron microscope with magnification of 25,000. The acceleration voltage was 10 kV. The samples were dispersed on an aluminum stub and coated with Au//Pt film. Temperature Programmed Reduction (TPR) experiments were carried out using a multifunctional apparatus (RXM-100 from Advanced Scientific Designs, Inc.). Prior to each TPR analysis, a 50 mg sample was calcined at 500oC for 90 min under flowing 20% O2/He (20 ml/min, ramp 5oC/min). The sample was then cooled to room temperature under flowing pure He (20 mL/min). H2-TPR of the catalyst was then carried out by ramping under 5 vol% of H2/Ar (20 ml/min) from room temperature up to 800oC (5oC/min). The hydrogen consumption was determined using a TCD with a reference gas of same composition as the reducing gas (H2/Ar). III - Results and Discussion 1. Catalyst characteristics Both the two perovskites were prepared by the reactive method, but NaCl was used as an additive for the preparation of LaCoO3 while no additive was introduced during the synthesis step of La0.9Ce0.1CoO3. Table 1 summarized several physical properties and preparative conditions of the solids. Table 1: Physical properties of catalysts Sample Tcal (oC) Crystal domain (nm)1 SBET (m2/g)2 Pore diameter (nm)2 Additive Impurity LaCoO3 250 9.8 56.9 12.7 NaCl Co3O4 La0.9Ce0.1CoO3 250 10.2 71.0 11.9 non La2O3, Co3O4 1: Calculated by the Scherrer equation from X-ray line broadening [11] 2: Determined from N2 desorption isotherm. Table 1 shows that the specific surface area of La0.9Ce0.1CoO3 sample is rather higher than that of the cobaltate. Meanwhile no significant difference in diameter of catalyst particles 554 between the two samples is observed (Table 1). This is consistent with the results of SEM technique. As seen from Figure 1 the catalyst is likely composed of uniform spherical particles. Such particles are further constituted of agglomerates of primary nanometric particles with diameters approximately of 10 nm as estimated from X-ray line broadening using Scherrer equation [9, 11]. The crystal domains are close to the pore diameter determined from N2 desorption isotherm, implying that their ground perovskites have somewhat porous structure. The porosity of the ground materials are assumedly built from the spaces of individual elementary nanoparticles. Similar structures were already reported by several groups for nanosized Co-based perovskites using the same preparation method [10, 12]. A B Figure 1: SEM photographs of LaCoO3 (A) and La0.9Ce0.1CoO3 (B) nanocrystalline perovskites after calcined at 250oC The crystallite phase identification of the solid solutions was accomplished by mean of XRD. X-ray diffraction spectra of all samples calcined at 250oC are displayed in figure 2. As seen from this figure, both the two ground catalysts show strong reflections of the perovskite phase with the rhombohedral structure. Besides the major phase of perovskites, some minor phases including Co3O4 in the case of LaCoO3 and Co3O4 + La2O3 in the case of Ce-containing sample were detected (figure 2 and table 1). The perovskite diffraction lines are rather broad, suggesting the formation of a nanophase [10]. This is good harmony with the SEM photographs illustrated in Fig. 1. Because the preparation of such samples is based on the solid-solid reactions between intimate mixtures of the constituent binary oxides (La2O3, Co3O4, CeO2) under milling conditions, the products have a limited degree of chemical homogeneity and usually consist of a small amount of unreacted oxide impurities [1, 5, 8, 10]. Therefore, the presence of Co3O4 and La2O3 on the ground perovskite surface is understandable. Figure 2 indicates the successful substitution of a small part of La3+ by Ce4+. No clear signals of CeOx are observable for the ground (La,Ce)CoO3 sample, implying that cerium is mostly in the perovskite structure. No significant change in rhombohedral structure between LaCoO3 and La0.9Ce0.1CoO3 perovskites is observed although 10% of the lanthanum in lattice is literally replaced by cerium. In this case, the perovkite structure is still well-preserved. Reducibility of La1-xCexCoO3 perovskites Temperature programmed reduction of hydrogen (H2-TPR) was carried out from room temperature to 800oC in order to investigate the 555 reducibility of the two ground perovskites. H2- TPR profiles of such materials displayed Figure 4 present a low-temperature peak at 410oC with a small back-shoulder at 440oC and a higher temperature peak at 705oC for LaCoO3 [13]. The amount of hydrogen consumed provides evidence of the reduction of Co3+ in perovskite lattice to Co2+ at 410oC. After this reduction step, the incompletely reduced Co2+ ions are assumed to be present in the perovskite framework. The higher tempature peak is ascribed to the reduction of Co2+ to metallic cobalt accompanied by the collapse of the perovskite structure. After the complete reduction, metallic cobalt phase is suggested to be very finely dispersed on lanthanum oxide. Meanwhile, H2-TPR curve of La0.9Ce0.1CoO3 perovskite show three distinct peaks at 310, 370 and 605oC, suggesting the occurrence of a multiple-step reduction. The reduction of Co3+ in the perovskite lattice to Co2+ takes place at 310oC. The small shaped-peak at 370oC, likely corresponding to the shoulder at 440oC for H2- TPR curve of LaCoO3, is assigned to the reduction of the extra-perovkite lattice cobalt (Co3O4) to lower oxidation states [14]. 50 350 650 950 1250 1550 20 30 40 50 602-Theta C ou nt s (a .u ) LaCoO3 La0.9Ce0.1CoO3 Perovskite Perovskite Perovskite Perovskite Perovskite Perovskite Co3O4Co3O4 La2O3 La2O3 La2O3 Figure 2: X-ray diffraction patterns of La1-xCexCoO3 0 20 40 60 80 50 200 350 500 650 800 Reduction temperature (0C) TC D S ig na ls (a .u ) La0.9Ce0.1CoO3 LaCoO3 410 440 750 310 370 605 Figure 3: H2-TPR of the two ground perovskites 556 In order to shed in light to this point, X-ray diffraction of both samples reduced at 400oC for 90 min were recorded. Figure 4 shows the presence of the main phase of perovskite with strong reflection intensities, indicating the stable perovskite structure under reduction conditions reported in figure 4. It should be paid attention to the disappearance of small reflections characterizing to the presence of cobalt oxide phase while signals of La2O3 are still detectable as compared with the spectra of their parent samples in figure 2. Instead, the appearance of very weak diffraction lines at 2θ = 44.3 and 51.5o is probably assumed to the formation of tiny metallic cobalt particles (Co) (figure 4). This is good agreement with the results of H2-TPR which indicate the reduction of Co3+ in perovskite lattice and Co3O4 oxide impurities to lower oxidation states at 310 and 370oC (figure 3). The higher temperature peak corresponds to the reduction of Co2+ in the perovkite intermediate phases of La0.9Ce0.1CoO3- x (x ≤ 2.5) perovskites to Co0 [13]. The reduction of La3+ and Ce4+ in the perovskite lattice likely occurs only at higher temperatures (> 800oC) [15]. It is clearly observed that the reduction temperature of La0.9Ce0.1CoO3 systematically shifts to lower temperatures as compared with that of cerium free-perovskite. Indeed, the presence of Ce4+ in the perovskite lattice decreases the reduction temperature of Co3+. In other words, cerium ions have the promoting effects on cobalt ion reduction. CeOx has been thoroughly reported to promote metal oxide reduction for Pd/CeO2/Al2O3, CuMgCeOx [16, 17]. The presence of cerium ions may be capable of playing a crucial role in the enhancement of reducibility of cobalt ions via the hydrogen spillover effects [18]. Another explanation may stem from some minor alterations in the perovskite structure. A small difference in the atomic radii and oxidation state between La3+ with trivalent (rLa = 1.061 Å) and Ce4+ with tetravalent (rCe = 1.034 Å) leads to a slightly distorted structure [5, 19]. This modified structure should be less thermal stability than the corresponding perovskite-type cobaltate (LaCoO3). In addition, a higher specific surface area of La0.9Ce0.1CoO3 is higher than the-one of LaCoO3 so that the diffusion of hydrogen reactant and the mass transport water product across lattice of the porous and on the external surface of the former (SBET = 70 m 2/g) become much easier than those of the latter (SBET = 56 m 2/g). All these factors make La0.9Ce0.1CoO3 perovskite less thermal stability under hydrogen atmosphere as demonstrated in figure 3. 0 150 300 450 600 750 20 30 40 50 602-Theta C ou nt s (a .u ) LaCoO3 La0.9Ce0.1CoO3 Perovskite Perovskite Perovskite Perovskite Perovskite Perovskite (Co) (Co) La2O3 La2O3 Figure 4: XRD patterns of La1-xCexCoO3 reduced at 400 oC under hydrogen atmosphere for 90 min 557 Fortunately, the decreased reduction temperature of cobalt ions in La0.9Ce0.1CoO3 leads to the prevention of the sintering of cobalt atoms in the perovskite lattice. The resultant relevant material may be present in the form of a perovskite-type metallic oxide that is promising catalyst for desirable applications [1, 6, 7, 20]. IV - Conclusion Two ground La1-xCexCoO3 perovskites have been successfully synthesized and characterized. The solid solutions have rather high surface area and consist of elementary nanometric particles. 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